<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        p{
            text-align: center;
        }
        span{
            color: red;
        }
    </style>
</head>
<body>
<p>
    <span id="time">5</span>秒之后跳转
</p>
<script>
    var time = document.getElementById("time");
      var a = 5;
    function fun() {
        a--;
        if(a<=0){
            location.href = "http://www.baidu.com"

        }
        time.innerHTML = a+"";
    }
    setInterval(fun,1000);
</script>
</body>
</html>